The present invention relates to an imaging system and more particularly to an ultrasonic imaging system.
Ultrasonic imaging systems are widely used in the medical diagnostic field for their ability to obtain the image of an object non invasively, i.e., by transmitting ultrasound to the object and processing its reflection. Conventional ultrasonic imaging systems have an array of ultrasound transducers or probes for generating ultrasound and receiving the ultrasound reflected off an object. Ultrasonic pulses from the array of ultrasonic transducers are focused to a desired point by controlling the timing of ultrasonic pulse generation at each of the transducers. FIG. 1 allows the timing control or ultrasound generation at an array of transducers in order to compensate propagation delay due to different distances from the transducers to a particular point. By sequentially delaying generation of ultrasonic pulse signals from the transducers, all the ultrasonic pulses simultaneously reach a point. Simultaneous reception of the reflected ultrasound from a particular point at the array of transducers is also made possible by sequentially adjusting receive timings of the transducers, where the greater the distance from a transducer to the point is the more receive delay is provided to the transducer. In order to obtain an accurate image of an object, transmit focusing to various points on the object is needed. But after transmitting ultrasonic pulses to be focused on a selected point, transmission to another point has to wait until all the reflected ultrasonic signals are received including one reflected from the farthest point. Increasing the number of transmit focal points has a drawback because it would also increase the amount of time required to obtain an image, thereby reducing the frame rate.
The frame rate in the case that each scan line transmits to focus on a single point is determined by the following equation.
1/FR=2D/vxc3x97N
wherein FR, D, v and N represent the frame rate, depth of scan, velocity of ultrasound transmission in the medium, and the number of scan lines respectively. As can be seen from the equation, the frame rate is inversely proportional to the number of scan lines, presenting one with a trade-off between the two variables. As a solution, a radial scan pattern to cover the whole area of diagnosis has been conventionally with ultrasound sequentially applied along N number of the scan lines to predetermined points. Beside the radial scan pattern as illustrated in FIG. 2, a parallel scan line pattern has also been widely used. With these scanning methods receive focusing is achieved only on the points along the scan lines, limiting collection of information on an object to the points of the scan lines. Display devices generally have pixels arranged in a matrix on their screens and each pixel should be provided with display data to form an image. FIG. 3 shows a scan converter 32, using the data collected with dynamic receive focusing, to generate display data for the pixels of a display device. The scan converter first stores data which are receive-focused from predetermined points on the scan lines and next converts it to a horizontal raster line display format used in most display devices. The information about a target object acquired by using the dynamic receive-focusing scheme is limited to focused points on the scan lines. These focused points do not necessarily coincide with actual pixel points of a display device that can represent the image of the target object (these actual pixel points corresponding to the pixel locations of the display device will simply be referred to as xe2x80x9cpixel pointsxe2x80x9d hereafter). Thus the scan converter has to perform interpolation to provide display data for all the pixels of the display device. For example, in the case of a radial scan pattern as shown in FIG. 4, because the distance between each pair of adjacent scan lines because greater as it is measured farther away from the transducers, the number of pixel points increases that do not have a one-to-one correspondence to points on the scan lines. As a result, display data for those pixel points can not be directly collected simply by the dynamic focusing. The scan converter determines display data for these pixels by interpolation using focused points on scan lines nearest to the pixels. Even in the case of parallel scan lines, some of the pixels in the display device may not correspond to those points on the scan lines. Thus a scan converter is still needed in a parallel scan-line ultrasonic imaging system.
FIG. 5 is a block diagram of an ultrasonic imaging system using a conventional dynamic receive focusing scheme. An array of transducers sequentially transmits ultrasound to be focused on desired points on the scan lines. After transmitting ultrasonic signals on a scan line, the transducers receive reflected ultrasonic signals and a beam-former focuses the received ultrasound from a plurality of points on the scan line. The function of the beam-forming part will be better understood by referring to FIG. 6. FIG. 6 describes computation of ultrasound propagation delay in the case of receive focusing and illustrates a case where M channels out of a total of M transducers are used for both transmission and reception, N channels being arranged in a curvilinear array with radius R(mm) and transmission angle xcex8max. When M channel/transducers disposed at the coordinates are used for receive focusing a point at distance Z on a scan line, the ultrasound propagation delay from the point (x, y) to an mth transducer is expressed as follows:             t      dm        =                            t                      i            ,            dm                          +                  t                      v            ,            dm                              =                        Z          v                +                              Z            m                    v                      ,            Z      m        =                                        (                          x              -                              x                cm                                      )                    2                +                              (                          y              -                              y                cm                                      )                    2                    
In order to discriminate a signal that was reflected from the point (x,y) from all the RF signals arriving at the mth transducer, a period of time from transmission to reception should be taken into account. In the above equation, the term tt,dm represents the time the ultrasound took from the start of transmission to reach the point and the term tr,dm represents the time the reflection took from the point (x,y) to the mth transducer. At the time of transmission, all the transducers are controlled to transmit respective ultrasonic signals such that they arrive simultaneously at a predetermined point. That the ultrasound travel times from each transducer to the point are the same may be assumed. By using the ultrasound propagation delay, each transducer reads the reflection signal it receives from the point and, by adding these reflection signals together, receive-focused data as to the point is obtained.
Referring back to FIG. 5, by repeating the receive-focus for a plurality of points along each scan line, an image of a target object can be obtained. These data are converted by a scan converter to appropriate values corresponding to pixels in a display device. The beam former (52) stores those data which are receive-focused at the points along the scan lines. In this process some of information in the reflection signals are lost. As shown in FIG. 4, because data associated with points between adjacent scan lines are not provided by the beam-former, the scan converter creates the image date, of these in-between points by interpolation using the image data of the adjacent scan lines. The interpolation, however, results in a distorted image. In order to reduce the distortion, the number of scan lines should be increased so that missing data between scan lines can be reduced but this presents a problem for an ultrasonic image system requiring real-time and high frame rate. Thus an ultrasonic image system which can prevent image distortion without increasing the number of scan lines has been needed.